In packing technology packages of a non-returnable kind have been used for a long time for the packing and transporting of, among other things, liquid foodstuffs such as milk, juice etc. A very large group of these known so-called non-returnable packages are manufactured from a material comprising a carrier layer of paper or cardboard with outer and inner coatings of thermoplastics. The material in these packages is often also provided with a further layer of other material, e.g. Al-foil or plastic coatings other than those mentioned here.
The choice of the composition of the packing material is based on the wish to create the best possible protection for the product which is to be packed, while at the same time giving the package sufficient mechanical strength and durability to enable it to withstand such external stresses as the package is subjected to in normal handling. To achieve mechanical rigidity, which on the one hand gives mechanical protection to the product and on the other hand makes it possible for the package to be dimensionally stable so that it can be handled without difficulty and manually gripped, the material in these packages is frequently provided with a relatively thick carrier layer of paper or cardboard. Such a material, however, possesses no tightness properties towards either liquids or gases and the rigidity of the material aimed at is quickly lost when it is subjected to moisture or liquid. To impart the required liquid tightness to the material, the carrier layer is provided therefore, frequently on both sides, with a liquid-tight coating of plastics material and, if this plastics material is a thermoplastics, the coating can also be used for sealing the plastics coatings to one another by so-called heat-sealing. In this manner packages can be sealed and made durably permanent in their intended shape by heat-sealing together thermoplastics-coated, overlapping material panels in a liquid-tight and mechanically durable sealing joint.
Non-returnable packages of the type referred to here are manufactured at present with the help of modern, rational packing machines which form, fill and close finished packages at a high rate of production from a web or from prefabricated blanks of a packing material. From a web, for example, packages are manufactured by joining together the longitudinal edges of the web in an overlap joint so as to form a tube. The tube is filled with the intended contents and is divided into closed package units by repeated flattening and sealing of the tube at right angles to the longitudinal axis of the tube. Subsequently the packing units are severed from one another by means of cuts in the transverse sealing zones, and they are given the desired geometrical shape by further folding and sealing.
During the manufacture of packages in the manner described above the laminated material is subjected to stresses which will be particularly great when the material is folded, since, owing to the relatively great material thickness of the carrier layer, a folding implies that the one plastics coating is subjected to a strong stretching, whereas the other plastics coating is compressed to a corresponding degree along the folding line. Thanks to a great extensibility of the plastic coatings, such folding of the material only rarely leads to breaks or other damage causing leakage in the extended plastics coating, but the problem is aggravated if the packing material also comprises an aluminium foil which compared with the plastics layers has a much smaller extensibility and consequently tends to fracture when the material is folded.
Even if a single 180.degree. folding of the material normally does not have any serious consequences, considerable difficulties arise if the material is to be folded along two crossing crease lines (so-called crosses). This is often the case in external sealing areas which occur on this type of package, whether they are manufactured from a web or from prefabricated blanks. The sealing generally is carried out by heating to melting the plastic coating facing towards the inside of the package along the edge zones which are to be sealed to one another, whereafter the heated plastic coatings are pressed to each other so as to form a sealing fin on the outside of the package held together through fusion of the material. Such a sealing fin comprises double material layers, and to ensure that it does not form an obstacle, the sealing fin frequently is folded down to lie flat against the outside of the package, which means that one of the material layers of the sealing fin undergoes a 180.degree. folding over, and that the package wall in the region of the folded-down sealing fin comprised three material layers, that is to say, has a threefold material thickness. Such a sealing fin often runs along one or more side faces of the package, and since these side faces during the shaping of, for example, parallelepipedic packages are subjected to a 180.degree. folding along a crease line at right angles to the sealing fin, the material thickness in certain regions of the package will go up to 6 times the laminate thickness. At this 180.degree. folding transversely to the sealing region, the material layers located outermost will be subjected to very strong tensile stresses with accompanying extensions and increased risks of crack formations connected therewith in the material. These tensile stresses frequently are so great that cracks occur not only in the aluminium foil included in the material but also in the thermoplastic coatings with accompanying leakage of the packed liquid, which can be absorbed readily by the carrier layer exposed owing to the crack formation, thus impairing the good rigidity in the material.
Disadvantages of the type described above, and which may be ascribed to a very large extent to the moisture-sensitive paper or cardboard layer of the conventional packing material, which at the same time has to be made relatively thick so as to impart the necessary mechanical rigidity to the packing container manufactured, may be avoided with the help of a packing material in accordance with the present invention.